Procedure for the independent reduction of acquisition thresholds and tracking of spread spectrum codes received in orbit

ABSTRACT

A procedure for independently reducing acquisition thresholds and tracking spread spectrum codes received in orbit by a receiver accessing an orbital navigator (internal or external to said receiver), wherein the receiver includes a phase loop and a code loop. The code loop that tracks the pseudo-random codes is &#34;impelled&#34; by a fine velocity aid and corrects the error between the real and calculated velocities. The code loop receiving pseudo-random codes is similarly &#34;impelled&#34; by the fine velocity aid, the code phase search being based on a phase prediction maintained by the fine velocity aid provided by the orbital navigator.

DESCRIPTION

1. Field of the Invention

The present invention relates to a procedure for the independentreduction of acquisition thresholds and tracking of spread spectrumcodes received in orbit.

2. State of the Prior Art

The invention combines three basic elements:

a spread spectrum signal receiver,

an on-board orbitography filter,

a technique for reducing thresholds using a fine radial velocity aid.

Each of these elements will be considered below.

The receiver may be any type of satellite-mounted equipment capable ofreceiving spread spectrum signals (see references [1], [2], [5]). Saidsignals may be transmitted by other orbiting satellites or fromterrestrial transmitters. The receivers may, for example, be of any ofthe following types:

GPS, GLONASS, GNSSA, GNSS2 receivers,

spread spectrum transponder,

DORIS NG receiver.

The GPS and GLONASS satellite constellations are described in references[3] and [4] respectively.

GNSS1 designates the geostationary equipment complementing GPS and/orGLONASS using the navigation packages of the INMARSAT 3 satellites.GNSS2 designates the future civil constellation of navigationsatellites.

DORIS NG designates a planned global radionavigation and spatialradiolocalization system, based particularly on the use of spreadspectrum signals transmitted by terrestrial beacons and received byorbiting satellites.

The orbitography filter is a digital processing system located in thereceiver, for example. It uses raw measurements taken by receivers, i.e.pseudodistance and pseudovelocity measurements relative to ground-basedor orbiting spread spectrum signal transmitters. These measurements areprocessed to give independently determined parameters for the orbitand/or the position and velocity of the carrier satellite. Definitionsof these measurements are given in reference [5]. The filter may be oneof the following types that are given as examples:

Kalman filter (see reference [6]),

single least squares filter,

recursive least squares filter.

This filter is also capable of determining the following parameters:

D_(i) =Distance between the satellite and transmitter No. i

ΔT_(i) =Time difference between receiver clock and clock of transmitterNo. i.

Vi=D_(i) =Radial velocity between the satellite and transmitter No. i.

ΔT_(i) =Relative deviation between clock of transmitter No. i andreceiver clock, where a "dot"above a variable signifies the driviativeof the variable with respect to time.

The clock can therefore estimate pseudodistances PD_(i) andpseudovelocities PV_(i).

PD_(i) =Di+C. ΔT_(i)

PV_(i) =Vi+C. ΔT_(i)

The given magnitudes can be estimated even if the signals transmitted bytransmitter No. i are not processed by the receiver and its associatednavigation filter provided the position, velocity and clock coefficientsof said transmitter can be estimated or established.

The orbital navigator estimates these pseudodistances andpseudovelocities to accuracies of δPD and δPV.

The orbital navigator can receive remote commands describing maneuversof the carrier satellite. These maneuvers can be described using theparameters ΔVx(t0), ΔVy(t0) and ΔVz(t0) where ΔV_(i) represents thecomponents of the velocity pulse at date t0.

Maneuvers are described with an accuracy noted as δVx, δVy, δVz for thethree axes. The overall accuracy of the description of the maneuver isδv where ##EQU1##

The technique of reducing the threshold using a fine radial velocity (orradial pseudovelocity) aid applies where receivers are fitted with oneor more phase loops coupled to one or more code loops. It is assumedthat said loops use digital technology.

When a signal is received with a signal-to-noise (C/NO) ratio lower thanthe usual reception threshold in assisted acquisition mode the carriersignal loop is open and the digitally-controlled oscillator (OCN) isdriven by an external radial velocity (or pseudovelocity) prediction.

Typical reception in assisted acquisition mode is illustrated inreference [1].

This velocity prediction must be fine and must be provided by a sensorother than the receiver. Typically said sensor may be an inertial unit,for example.

This type of technology is normally used for tracking low equivalentC/NO ratio GPS signals received by military GPS receivers (C/A and Pcode) embarked on fighter aircraft fitted with inertial units. Thistechnique is described as being "code only" since only the pseudo-randomcode is tracked down to very low thresholds.

SUMMARY OF THE INVENTION

The present invention relates to a procedure for independently reducingacquisition thresholds and tracking spread spectrum codes received inorbit.

More specifically, the present invention relates to a procedure forindependent reducting of acquisition thresholds and tracking spreadspectrum codes received in orbit by a receiver accessing an orbitalnavigator internal or external to said receiver, wherein, the receiverincludes a phase loop and a code loop, with the code loop trackingpseudo-random codes and is "impelled" by the velocity aid to correct theerror between the real and calculated pseudovelocities. The code loopreceives pseudo-random codes and is similarly "impelled" by the finevelocity aid, wherein the code phase search is based on a phaseprediction maintained by said velocity aid.

The procedure of the present invention includes the following steps:

receiving, at a receives aids required for normal assisted acquisition,enabling the receiver to receive all signals with a C/No ratio such asC/No>(C/No)_(a), where (C/No)_(a) is a reception threshold in normalassisted acquisition mode, and

lowering the thresholds of pseudo-random codes to a value (C/No)_(avf),where (C/No)_(avf) is the reception threshold of pseudo-random codes inreception mode assisted by a fine velocity aid, said fine velocity aidbeing provided by the orbital navigator.

That procedure may further include a preliminary step in which thereceiver cold starts without any internal or external assistance ormessage and receives all signals with a C/No ratio such asC/No>(C/No)_(na) where (C/No)_(na) is the reception threshold innon-assisted mode.

In a maneuver to control the orbit of the satellite the orbitalnavigator receives a description of said maneuvers and updates thevelocity aid supplied by the navigator. In order to continue receivingpseudo-random codes with low C/No ratios during maneuvers, the followingcondition must be satisfied first and foremost: ##EQU2## where δPV isthe uncertainty of the prediction of the pseudovelocity provided by thenavigator in the absence of maneuvers, where B_(FI) is the predetectionband, C is the speed of light and f_(i) the frequency of the carriersignal transmitted by transmitter No. i.

When a pseudo-random code is tracked with a C/No ratio such as(C/No)_(avf) <C/No<(C/No)₃, the updates of the characteristic parametersof the transmitters are sent to the receiver by means of remote commandsexternal to the system.

The procedure of the invention includes reducing reception and trackingthresholds of spread spectrum codes received by satellite receiversfitted with an on-board orbitography filter. That threshold reduction isachieved independently by the receivers using the invention. Theresulting threshold reduction may be spectacular.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a receiver with a threshold reduced by anexternal fine velocity aid,

FIG. 2 is a block diagram of a receiver incorporating the apparatusaccording to the invention, and

FIG. 3 illustrates a variant of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a spread spectrum receiver where the threshold is reducedby an external fine velocity aid. Only the digital architecture isshown.

Receiver 10 comprises an RF module 11 connected to an antenna 12 whoseoutput signal is fed into a correlator 13 whose output is in turnconnected to a phase loop discriminator 14 followed by a loop filter 15and a selector switch 16. A carrier signal digitally-controlledoscillator (OCN) 17 transmits a local carrier signal Fi that is in phaseand quadrature with discriminator 14 and is connected to anotherterminal of selector switch 16. The selector switch 16 also receives asignal from an external velocity sensor 18 that may, for example, be ainertial unit.

The carrier signal digitally-controlled oscillator (OCN) transmits anOCN velocity carrier signal to a first input of a summing integrator 19whose output is connected to an OCN code module 20, and to a local codegenerator 21. The generator 21 is connected to correlator 13, supplyingit with the local code in phase, and to a code loop discriminator 22,supplying it with the local lead and lagging codes, a loop filter G(p)23 being disposed between the output of discriminator 22 and a secondinput of summing integrator 19.

Both phase loop 33 and code loop 34 are therefore present.

Tracking

The code loop that tracks the pseudo-random codes is "impelled" by thevelocity aid. In other words, the digitally-controlled oscillator (OCN)of this loop causes the local code phase to vary with a velocity equalto the external velocity prediction aid. The "impelled" code loop thuscorrects the error between the real and calculated velocities.

This loop should be of an order that is sufficient to maintain theslaving of the digitally-controlled oscillator (OCN), allowingpseudodistance measurements to be generated.

Reception

The code loop that receives pseudo-random codes is also "impelled" bythe fine velocity aid. The phase search of the received code is based ona phase prediction (distance prepositioning) maintained by the externalvelocity aid.

The search zone of the received code phase is smaller than for receptionin standard mode. The same applies to the search zone of the receivedcarrier signal frequency. The principle described operates if theuncertainty of the Doppler prediction ΔF_(p) is less than thepredetection band B_(FI).

The uncertainty δPV of pseudovelocity prediction must therefore complywith the following relations: ##EQU3## where C is the speed of light andf_(i) the frequency of the carrier signal transmitted by transmitter No.i.

Therefore, since zones of uncertainty in Doppler and distance modes areweaker than normal, energy search can be carried out with a much slowerlocal code scan speed than normal for the same search time, noted as T.The reception threshold is thereby lowered. Techniques for rejectingspurious reception must be implemented if several pseudo-random codeswith disparate C/No ratios are received simultaneously.

The following C/No ratios are defined:

(C/No)_(na) =Reception threshold in non-assisted mode

(C/No)_(a) Reception threshold in normal assisted mode

(C/No)_(avf) =Reception threshold of pseudo-random codes in receptionmode assisted by a fine velocity aid

Details of thresholds (C/No)_(na) and (C/No)_(a) are given in reference[1].

Threshold (C/No)_(avf) is a function of several parameters.

    (C/No).sub.avf =g(T;δPV;B.sub.FI)

The invention is characterized by the following procedure that isimplemented in a spread spectrum receiver in a satellite equipped withan orbital navigator.

Stage 1 (optional)

The receiver cold starts without any internal or external aid ormessage. It receives all signals with a C/No ratio such as:

    C/No≧(C/NO).sub.na

The first signals received may allow the receiver to:

receive messages used to establish position and/or velocity and/or clockcoefficients of transmitters No. i,

bring the orbital navigator into convergence using the firstpseudodistance and pseudovelocity measurements taken.

This first step is necessary in fully-independent space missions.

Stage 2

The receiver receives aids required for normal assisted acquisition.Said (coarse) aids are fairly imprecise, consisting of the followingtype of data:

1) Receiver clock date and time.

2) Positions/velocities (or possibly orbital parameters) of thetransmitters.

3) Positions/velocities or orbital parameters of the carrier satellite.

These aids may originate wholly or partly from stage 1. In this casethey are internal to the receiver (e.g. positions/velocities may betransmitted by said transmitters) and independence of operation ispreserved.

Where some or all of these coarse aids are communicated to the receiverby means of external remote commands, the receiver can no longer be saidto be independent.

If stage 1 is omitted, said aids are inevitably external to thereceiver.

Such coarse aids enable the receiver to receive all signals with a C/Noratio such as:

    C/No>(C/No).sub.a

The number of pseudovelocity and pseudodistance measurements is greaterthan in stage 1 since:

    (C/No) a>(C/No).sub.na.

The number of such measurements is assumed to sufficient to bring theorbital navigator supplying the orbital parameters of the carriersatellite into convergence with greater accuracy than in stage 1.

Stage 3

Once stage 2 is complete, it is assumed that the accuracy of theparameters output by the orbital navigator and the parameterscharacteristic of the transmitters is compatible with the accuracy ofthe velocity aid required to further lower the reception thresholds ofthe pseudo-random codes to the value (C/No)_(avf) .

In contrast with systems using the known art, the fine velocity aid isprovided by the orbital navigator integrated into the receiver. This aidis thus internal and independence is preserved, as shown in FIG. 2.

The accuracy of the navigator may thus be improved. Moreover, wherethere is progressive deterioration of contact with the transmitters,deterioration of this accuracy may be limited.

The situation is then:

    (C/No).sub.avf <(C/No).sub.a

Stage 4

In maneuvers aimed at controlling the orbit of the satellite, theorbital navigator receives a description of said maneuvers and updatesthe velocity aid provided by the navigator.

In order that pseudo-random codes with low C/No ratios may continue tobe received during maneuvers the following condition must be satisfiedfirst and foremost: ##EQU4## where δPV is the pseudovelocity predictionprovided by the navigator where no maneuvers are present.

Stage 5

When a pseudo-random code is tracked with a C/No ratio such as:

    (C/No).sub.avf <C/No<(C/No).sub.a

demodulation of message data sent by the transmitters is not possible.

Such demodulation should, in fact, be effected by the carrier signalloop (phase loop). This loop is, in fact, open when the C/No ratiocomplies with the double inequality cited above.

Updates of the characteristic parameters of the transmitters (positionsand/or velocities and/or clock coefficients) must therefore becommunicated to the receiver using external remote control means. Forexample, in a GPS or GLONASS receiver these parameters may be theephemerides of the constellation used.

FIG. 2 shows a spread spectrum satellite receiver 29 comprising an RFmodule 30 receiving a signal from an antenna 31 and connected to a firstinput of a correlator 32 followed by a phase loop 33 connected to a codeloop 34 that also receives the output signal from RF module 30 and whoseoutput is connected to a second input of correlator 32, and anintegrated orbital navigator 35 that receives data and pseudovelocitiesfrom the phase loop provided (C/No)>(C/No)_(a) and that sends the phaseloop a fine velocity aid and receives pseudodistances from the codeloop.

The integrated orbital navigator receives the description of thesatellite maneuvers and external data.

FIG. 3 shows a variant of the invention wherein the orbital navigator isintegrated into an on-board computer 36 present in the satellite.

EXAMPLES OF APPLICATIONS

The applications of the procedure according to the invention to on-boardreception of spread spectrum signals concern situations in which thereis poor communication between the transmitters used and said satellites.

Such applications may, for example, relate to:

types of receiver

Navigation using a satellite constellation receiver (e.g. GPS, GLONASS).

Navigation using a spread spectrum transponder. Reception may be poor atthe beginning and end of overflight of a remote control/remotemeasurement (TM/TC) station.

Navigation using a receiver of spread spectrum signals transmitted by agroup of terrestrial beacons equipped with hemispherical antennas. Thepower transmitted by said beacons is assumed to be optimized for use bylow-orbit satellites. Reception is thus assumed to be poorer forsatellites in, for example, geostationary orbit.

Reception in orbit of spread spectrum signals in environments scrambledby unwanted radioelectric transmitters. The equivalent C/No ratio ofsignals received is reduced compared with a non-scrambled environment.Reception is deteriorated and the procedure according to the inventionmay be necessary.

Orbits

Navigation using a GPS or DORIS NG receiver in geostationary transferorbit. The geostationary transfer orbit may be any of the followingtypes:

normal geostationary transfer orbit,

supersynchronous orbit,

subsynchronous orbit,

drift orbit.

These types of navigation may be performed using two low-gain antennasif the reception threshold of the signals is low (see reference [7]).

Navigation using a receiver in high-apogee orbit, for example any of thefollowing (see reference [8]):

Molniya orbit,

Tundra orbit,

Archimedes orbit.

Navigation using a receiver in circular orbit for a constellation ofnavigation satellites with a period of the order of twelve hours.Terrestrial beacons with hemispherical antennas are suitable for thisapplication.

Navigation using a low-orbit receiver connected to one or more receptionantennas that are low-cost and therefore not optimized, but efficientenough to enable the receiver to effect stage 2 described above.

REFERENCES

[1] "Orbital Navigation with a GPS Receiver on the HETE Spacecraft" (IONGPS January 1994, pages 645-656).

[2] "ESA Dual-Standard S-Band Transponder: A Versatile TT&C Equipmentfor Communications Via a Data Relay Satellite or Directly with theGround Network" by J. L. Gerner (42nd Congress of The InternationalAstronautical Federation, 5-11 Oct. 1991).

[3] "Accord de standardisation; caracteristiques due systeme mondial dedetermination de la position NAVSTAR (GPS)" (Standardization Agreement:Characteristics of the NAVSTAR Global Positioning System (GPS)), (NATO,STANAG 4294.

[4] "GLONASS Approaches Full Operational Capability (FOC)" by P. Daly(ION GPS, September 1995).

[5] "Techniques et technologies des vehicules spatiaux module 6.Localisation spatiale" (Techniques and technology of space vehicles,module 6. Space tracking), Editions Cepadues.

[6] "Low-Orbit Navigation Concepts" by H. James Rome (vol. 35, No. 3,Fall 1988, pages 371-390).

[7] "GPS Techniques for Navigation of Geostationary Satellites" by P.Ferrage, J. L. Issler, G. Campan and J. C. Durand (ION GPS, 12-15September 1995).

[8] "Applicability of GPS-Based Orbit Determination Systems to a WideRange of HEO Missions" by J. Potti, P. Bernedo and A. Pasetti, (ION GPS,12-15 September 1995).

I claim:
 1. A method for independently reducing acquisition thresholdsand tracking spread spectrum codes received in orbit by a receiveraccessing an orbital navigator, wherein the receiver includes a phaseloop and a code loop, the method comprising the steps of:receiving, inthe receiver from the orbital navigator, a fine velocity aid requiredfor normal assisted acquisition, enabling the receiver to receive allsignals with a signal-to-noise (C/No) ratio when C/No>(C/No)₃, where(C/No)₃ is a reception threshold in normal assisted acquisition mode:tracking, using the code loop, pseudo-random codes; correcting an errorbetween real and calculated velocities by using the fine velocity aid,searching for a code phase based on a phase prediction maintained by thefine velocity aid provided by the orbital navigator; and loweringthresholds of the pseudo-random codes to a value (C/No)_(avf), wherein(C/No)_(avf) is a reception threshold of pseudo-random codes inacquisition mode assisted by the fine velocity aid provided by theorbital navigator.
 2. The method as claimed in claim 1, furthercomprising a preliminary step of cold starting the receiver without anyassistance or message and receiving signals with a C/No ratio whenC/No≧(C/No)_(na), where (C/N)_(na) is the reception threshold innon-assisted mode.
 3. The method as claimed in claim 1, furthercomprising the steps of:receiving in the orbital navigator, during amaneuver to control the orbit of the receiver, a description of themaneuver: and updating the velocity aid supplied by the orbitalnavigator, wherein the following condition is satisfied: ##EQU5## whereδPV is an uncertainty of a prediction of the pseudovelocity provided bythe orbital navigator in an absence of the maneuver, and where δV is anaccuracy of the description of the maneuver.
 4. The method as claimed inclaim 1, wherein when a pseudo-random code is tracked with a C/No ratiowhen (C/No)_(avf) <C/No<(C/No)_(a), then updates of characteristicparameters of transmitters are sent to the receiver using remotecommands external to the system.
 5. The method as claimed in claim 1,wherein the step of receiving comprises receiving the fine velocity aidfrom the orbital navigator that is internal to the receiver.
 6. Themethod as claimed in claim 1, wherein the step of receiving comprisesreceiving the fine velocity aid from the orbital navigator that isexternal to the receiver.